Contemporary fluid dispense systems are well suited for dispensing precise amounts of fluid material at precise positions on a substrate. A pump transports the fluid to a dispense tip, also referred to as a “pin” or “needle”, which is positioned over the substrate by a micropositioner, thereby providing patterns of fluid on the substrate as needed. As an example application, dispense tips can be utilized for depositing precise volumes of adhesives, for example, glue, resin, or paste, during a circuit board assembly process, in the form of dots for high-speed applications, or in the form of lines for providing underfill or encapsulation.
FIG. 1 is a perspective view of a conventional dispense tip 24. The dispense tip 24 includes a body 26 and a hollow neck 28. The body 26 attaches to a pump 22, for example by means of a thread, which controls the amount of fluid to be dispensed. The neck 28 is typically a hollow cylinder having a first end 31 which is positioned to overlap with an aperture formed in the body 26, and a second end 30 at which the fluid is dispensed.
As shown in the close-up perspective view of FIG. 2, the neck 28 is formed by rolling a flat portion of machined metal into a cylindrical form. A seam 40 is welded along the longitudinal axis, to seal the edges of the flat portion, using conventional seam welding techniques. In precision tips, the inner diameter of the opening at the second end 30 may be on the order of 0.030 inches in diameter. The thickness of the walls 32 may be on the order of 0.010 inches. A hole 29 is bored into the tip body 26, and the neck 28 is aligned with, and pressed into, the hole. As a consequence of rolling and welding, the inner diameter of the neck is often unpredictable due to inner collapse.
When fluid is released at the opening 30, a high degree of surface tension on the substrate is desired, such that the substrate receives and pulls the fluid from the tip 24. It is further desirable to minimize the surface tension of the neck 28 interface such that when the pin retracts from the substrate, dispensed fluid properly remains on the board. However, a certain degree of surface tension in the neck exists due to the thickness of the walls 32 of the neck 28 at the opening 30.
It has been observed that the surface tension, or “land”, at the opening 30 of the neck 28 can be reduced by tapering the outer diameter of the neck 28 to a sharp point. As shown in FIG. 3, the distal end 30 of the neck 28 is sharpened using a surface grinder 42. The neck 28 is positioned perpendicular to the motion of the grinder 42 as shown, to thereby generate a taper 36, or bevel, on the distal end of the neck 28. The tapered portion 36 varies in thickness from the outer diameter of the neck 28 at position 37A to a sharpened point 37B at the opening 30. For the example given above, by providing a taper 36, the amount of land at the opening may be reduced from 0.010″ of contact about the perimeter of the opening, to 0.001″ of contact. In this manner, the surface tension at the junction of the pin and fluid is highly reduced, leading to a higher degree of dispensing precision.
As shown in the close-up perspective view of FIG. 4, as a consequence of formation of the taper 36 in the manner described above, with the neck 28 positioned substantially perpendicular to the grinding wheel 42, tooling scars, in the form of radial rings 38, can form on the taper 36 due to surface variations in the grinding wheel 42. These rings 38 provide ledges or shelves that can lead to additional surface tension on the taper 36, which, in turn, capture fluid material when the tip is released from the substrate following a fluid deposit. This, in turn, can cause fluid to be dispensed inconsistently on the substrate during subsequent deposits, leading to inaccurate results.